Deposition of tungsten nitride

ABSTRACT

Methods for depositing a tungsten nitride layer are described. The methods form a tungsten nitride layer using a carefully controlled deposition technique such as pulsed nucleation layer (PNL). Initially, a tungsten layer is formed on a substrate surface. The tungsten layer is then exposed to a nitriding agent to form a tungsten nitride layer. Methods of forming relatively thick layers of involve repeated cycles of contact with reducing agent, tungsten precursor and nitriding agent. In some cases, the cycle may also include contact with a dopant precursor such as phosphine or arsine.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a divisional application of and claims priority fromU.S. patent application Ser. No. 10/690,492, filed on Oct. 20, 2003, nowU.S. Pat. No. 7,005,372 entitled “DEPOSITION OF TUNGSTEN NITRIDE,” byKarl B. Levy et al. as inventors; which claims priority under 35 USC119(e) from U.S. Provisional Patent Application No. 60/441,834 filedJan. 21, 2003, naming Fair et al. as inventors, which are incorporatedherein by reference in its entirety. This application is also related tothe U.S. application Ser. No. 09/975,074 (issued as U.S. Pat. No.6,635,965) filed Oct. 9, 2001, entitled ULTRA THIN TUNGSTEN LAYER WITHIMPROVED STEP COVERAGE, by Lee et al., and U.S. application Ser. No.10/649,351, filed Aug. 26, 2003, entitled METHOD FOR REDUCING TUNGSTENFILM ROUGHNESS AND IMPROVING STEP COVERAGE, by Lee et al., which isincorporated herein by reference for all purposes.

FIELD OF THE INVENTION

This invention pertains to pulsed nucleation layer (PNL) methods fordepositing tungsten nitride. Specifically, the invention pertains tomethods that deposit tungsten nitride on partially fabricatedsemiconductor devices. The invention is particularly useful forapplications that require metal or metal nitride deposition ondielectrics, metals, silicides, and silicon with good adhesion,excellent step coverage, and low processing temperatures (e.g., 400° C.or lower).

BACKGROUND

Tungsten nitride is used in several applications for semiconductordevice fabrication. As deposited by traditional means such as PVD andPECVD, tungsten nitride provides relatively low resistivity, goodadhesion to dielectric films, and is a good diffusion barrier. A keylimitation that has prevented wider application of WN in the past hasbeen poor step coverage in high aspect ratio trenches, vias andcontacts.

To be successful in nanometer scale applications, tungsten nitride mustbe deposited thinly and conformally in high aspect ratio features.Conventional physical vapor deposition (PVD) techniques are not able tomeet these criteria. To accomplish thin conformal coverage, chemicalvapor deposition (CVD) methods are typically considered. A conventionalCVD process involves the simultaneous introduction of gas phasereactants, including tungsten precursor (typically tungsten hexafluoride(WF₆)) and a nitrogen containing gas (e.g., N₂), near a heated wafersurface while a vacuum is applied to the system. The reaction is drivenby the energy provided by the heated wafer and the free energy change ofthe chemical reaction. The growth of the tungsten nitride film continuesas long as the reactants and energy source are available.

Although standard tungsten nitride CVD techniques can provide good stepcoverage and adequately fill low aspect ratio features (e.g., <5:1aspect ratio), as semiconductor fabrication technology approaches thenanometer scale, the demands for step coverage and gap filling arebecoming more stringent and CVD may not be suitable for the task.Traditional plasma-enhanced CVD of tungsten nitride has relatively poorstep coverage for a CVD process (<50% SC in a 5:1 aspect ratiocylindrical contact). This is not adequate for the demands of currentand future semiconductor technology with aspect ratios exceeding 10 to 1and critical dimensions less than 100 nanometers. CVD and particularlyPNL or ALD tungsten processes (as opposed to tungsten nitride processes)can provide the very high step coverage and conformal depositionrequired for modern semiconductor devices, but will not adhere directlyto dielectric surfaces. Tungsten now requires an adhesion layer such asTiN before deposition on dielectric surfaces. Finally, the highdeposition temperatures required by many TiN deposition techniques (e.g.PECVD-TiN from TiCl₄) require high deposition temperatures that areincompatible with low-K dielectrics or nickel silicide.

What are therefore needed are improved methods for depositing tungstennitride.

SUMMARY OF THE INVENTION

The present invention provides methods for depositing a tungsten nitridelayer on a substrate, where the methods provide good tungsten nitrideadhesion to the substrate, fine control over deposition thickness, andgood step coverage over high aspect ratio regions of the substrate. Toaccomplish this, the invention provides a pulsed nucleation layer methodfor depositing tungsten nitride. Generally, the invention employs atleast the following operations (performed in various orders): (i)providing a layer of reducing agent on a substrate surface, (ii)contacting the substrate surface with a tungsten containing precursor toform a tungsten layer on the substrate, and (iii) nitriding the tungstenlayer to form tungsten nitride.

In many cases, the substrate is a semiconductor wafer or a partiallyfabricated semiconductor wafer. Applications of the invention includeusing tungsten nitride as (or as part of) a copper diffusion barrier, agate electrode, a capacitor electrode, and a diffusion barrier and/oradhesion layer in tungsten plugs, a sacrificial hard mask for use indual damascene copper interconnect formation, and a light shield for CCDdevices. Of course, the invention can be used in other applicationsrequiring high-quality tungsten nitride layers. In many of theseapplications, the tungsten nitride is deposited at least partially overa dielectric material.

In some preferred embodiments, the reducing agent is a boron-containingagent, preferably a borane such as diborane (B₂H₆). The borane reducingagent may be introduced as a gas phase reactant that decomposes on thesubstrate surface, creating a boron containing “sacrificial layer.”Preferably, this sacrificial layer is between about 3 and 20 angstromsthick and deposited at a temperature between about 200 and 400 degreesC.

In alternative embodiments, the reducing agent is a silane or othernon-boron-containing reducing agent. In such embodiments, the reducingagent may be introduced to the substrate to form an adsorbed orsaturated layer prior to introducing the tungsten-containing precursor.Alternatively, the tungsten-containing precursor can be introduced tothe substrate before the reducing agent. If the tungsten-containingprecursor is introduced before the reducing agent, a thin layer oftungsten-containing precursor forms on the substrate and is subsequentlydecomposed to form tungsten upon contact with the reducing agent.

In some embodiments, the tungsten-containing precursor is WF₆, WCl₆, orW(CO)₆. Of course, other tungsten-containing precursors suitable forreduction to elemental tungsten can be employed, including a combinationof tungsten-containing precursors (whether gases, liquids, or solids).Also, any suitable nitriding agent can be used. Examples include N₂,NH₃, NF₃, N₂H₆, and combinations thereof.

In certain embodiments of the invention, a gas purge is employed afterexposure to one or more of the reactants; e.g., the reducing agent, thetungsten containing precursor, and the nitriding agent. In many cases, agas purge is employed after each of the reactants is introduced. The gaspurge clears the regions near the substrate surface of residual gasreactants that could react with fresh gas reactants for the nextreaction step. In some embodiments of the invention, it is preferable totreat the tungsten layer with hydrogen or Ar—H₂ plasma before thenitridation step to remove the halogen byproducts, unreacted halogenreactants, or other undesirable gases before the introduction of thenitriding agent.

In another aspect of the invention, the tungsten nitride is deposited ina dedicated tungsten nitride module with one or more depositionstations. The tungsten nitride module contains a wafer preheat station,and a substrate preclean station. The preclean module provides featuresfor a reactive preclean that makes use of a fluorine based cleanchemistry generated by dissociation of a fluorine containing reagentusing an inductively coupled plasma. Further, the wafer preclean stationor another station in the tungsten nitride deposition module possessesfeatures for passivating the substrate after substrate precleaning.Preferably, the module for tungsten nitride deposition is vacuumintegrated with a module dedicated for pulsed nucleation of tungsten orCVD of tungsten.

In one embodiment, the method also passivates the substrate by means ofone or more of the following: (a) hydrogen exposure; (b) exposure to aremote H/H₂ plasma; (c) exposure to direct H/H₂ or Ar/H/H₂ or a RFplasma; (d) exposure to WF₆; (e) exposure to H₂ or H/H₂ plasma and NH₃in series or simultaneously; and (f) exposure to oxygen.

Yet another aspect of the invention provides a method of forming atungsten nitride layer on a substrate, which method is characterized bythe following sequence: (a) positioning the substrate in a depositionchamber; (b) depositing a one or more layers of pulsed depositiontungsten on the semiconductor wafer; (c) depositing one or more layersof pulsed deposition tungsten nitride on the one or more layers oftungsten; and (d) optionally repeating (b)-(c) to generate either abilayer of W—WN or a multi-layered structure of multiple tungsten andtungsten nitride layers. The resulting composite film can have variouslayered structures. In one embodiment, the bottom layer of the W—WNcomposite film is a tungsten layer. In another embodiment, the bottomlayer of the W—WN composite film is a WN layer. In one specificembodiment, the ratio of W and N atoms are present in the W and WNlayers in a ratio of approximately 2-to-1, such that stoichiometric W₂Nis formed directly, or indirectly by a heat treatment.

A specific aspect of the invention pertains to depositing tungstennitride over an inter-metal dielectric material to act as a copperdiffusion barrier. In these applications, a tungsten layer may bedeposited over the tungsten nitride layer. Thereafter, copper isdeposited on the tungsten nitride or W—WN barrier to create a film stacksuitable for single or dual damascene copper interconnect formation.

If a metallic tungsten layer is employed as part of a WN—W barriersystem, it may be deposited using pulsed nucleation layer (PNL) methods,atomic layer deposition methods, or CVD methods (e.g., using WF₆ and H₂or SiH₄ or any combination thereof). In conventional Damasceneprocesses, the copper layer is provided first as a copper seed layer,which can be deposited using sputter deposition or electroless platingmethods. After the copper seed layer is deposited, a bulk copper layercan be deposited over the seed layer using electrolytic plating methods.

Another aspect of the invention pertains to use of tungsten nitride,which may be used in combination with a tungsten layer on the tungstennitride, to form a gate electrode. In another application, a tungstennitride layer or a tungsten nitride-tungsten bilayer is used to formcapacitor electrodes for DRAM or other storage devices. These and otherfeatures and advantages of the invention will be described in moredetail below with reference to the associated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow diagram illustrating relevant operationsemployed in the present invention. A thin, conformal tungsten metallayer is formed onto a substrate, followed by nitridation of thetungsten metal to form tungsten nitride.

FIGS. 2A-2D are sequential cross-sectional view illustrations of apulsed nucleation layer deposition mechanism.

FIG. 3 is a schematic diagram showing the basic features of an apparatussuitable for practicing the current invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Introduction

As indicated, the present invention provides methods for depositing atungsten nitride layer, especially for applications in which thetungsten nitride is deposited over dielectric surfaces and wherein thin,conformal and adhesive layers are required. Preferred methods involvepulsed nucleation layer (PNL) deposition techniques, which will bedescribed in detail below.

One preferred approach to the PNL process involves first depositingdiborane (or other boron-containing precursor) on a substrate surface toform a “sacrificial” boron-containing layer. This sacrificial layersubsequently reacts with a tungsten precursor to form tungsten. Thediborane deposition process is not a conventional self-limiting ALD typedeposition process. Rather, the diborane reacts on the dielectricsurface to decompose into a boron film. The reaction can proceed as longas the substrate is continually exposed to diborane. However, to ensurethat a limited amount of tungsten is actually formed in the subsequentstep, the diborane deposition is preferably limited to a thickness ofbetween about 3 and 10 angstroms. This may correspond to about one ortwo monolayers of boron.

Note that the terms boron layer (or “film”) and boron-containing layerencompass pure elemental boron as well as various boron compounds suchas boron hydride, and mixtures or other combinations of such compoundswith each other and/or with elemental boron. The term “elemental boron”as used herein likewise encompasses pure elemental boron as well ascombinations of elemental boron with some amount of other material suchas any one of many boron-containing compounds.

In the second operation of the process, the boron layer is exposed to atungsten precursor, which is reduced by the boron to form tungsten. Inthe third operation, the tungsten layer is converted to tungsten nitrideby contact with a nitriding agent. Generally, one wants all of thetungsten to be converted to tungsten nitride. A preferred product isstoichiometric W₂N, although WN, WN₂, and various other stoichiometriesare also covered by this invention. The final film may also containvarious hydrides and/or amines, for example. If some amount of tungstenremains unconverted, it can limit the adhesiveness of the layer to theunderlying dielectric. Hence, it can be important that the amount oftungsten present in any cycle of the PNL reaction be sufficiently smallthat it is entirely converted to nitride during the nitriding step. Theamount of tungsten produced is limited by the amount of boron that hasbeen previously formed as a sacrificial layer on the underlyingsubstrate. Hence, the amount of diborane deposited and reacted to formboron effectively limits the amount of tungsten, which in turn, ensuresthat all tungsten can be converted to nitride in a single operation.

After the three operations are completed, a very thin layer of tungstennitride results. Thereafter, the three operations are repeated inmultiple cycles until a desired thickness of tungsten nitride is formed.In subsequent cycles, different reactants can be employed. Most notably,the diborane can be replaced with some other reducing agents such assilane or another silicon hydride.

The process conditions employed at the various steps of the cyclicaloperation can be widely varied. Relevant process conditions includepressure, temperature, dose, concentration, and time. The nitridingoperation offers the most flexibility in choice of these conditions. Thediborane operation is preferably performed at a relatively hightemperature (e.g., greater than 250 degrees centigrade, preferablybetween about 200 and 400 degrees centigrade) in order to ensure that asufficient quantity of boron is formed on the surface in a reasonableamount of time. Regarding other parameters, generally the process stepsare performed at a pressure of between about 0.1 and 300 torr and thedoses are defined by the flow rates and exposure times. These parametersare described in more detail below.

The PNL process may be preceded by various substrate pretreatments suchas a degas operation, an anneal, and/or a preclean, (e.g., a mildsputter etch in argon and/or hydrogen). These pretreatments have variouspurposes such as desorbing water vapor and removing surface oxidationfrom electrically active regions of semiconductor devices. They will bedescribed in more detail below. Also, the PNL process may includevarious post-treatments such as depositing a metallic tungsten layer ontop of the tungsten nitride in order to create a WN—W bilayer.

In a typical scenario, the PNL-WN deposition process is preceded by awafer degas/preheat and preclean. Traditional wafer degas isaccomplished with an independent high-vacuum chamber in which the waferis heated and outgassing species are pumped away. Existing and well knowwafer preclean strategies include direct Ar and Ar—H₂ sputter etch tophysically remove contaminants and surface oxidation prior to barrierdeposition.

One implementation of PNL-WN includes an in-situ preheat and reactivewafer preclean. Wafer preheat is used to drive moisture and othercontaminants out of the wafer surface. Wafer preclean may beaccomplished by brief exposure of the wafer to a tightly controlled doseof atomic or molecular fluorine. In one implementation the fluorine isgenerated by dissociation of NF₃ into F and N₂ by means of aninductively coupled plasma (ICP), although other fluorine source gasses(e.g. F₂, CF₄, C₂F₆, ClF₄, etc) and other dissociation techniques areequally applicable and within the scope of this invention. The fluorinespecies generated during wafer preclean react with native oxides andother residues, generating volatile products which desorb from the wafersurface and are pumped away.

In one example, a wafer preclean apparatus includes an NF₃ divert linewhich allows the NF₃ flow from a MFC (mass flow controller) to bediverted directly to a process foreline of the deposition chamber untilthe flow is fully stabilized prior to introduction into the waferpreclean station of the chamber. A parallel outlet valve allows acontrolled burst of NF₃ to be delivered at the onset of NF₃ delivery tothe deposition chamber.

As explained below, all gas flow, valve, and plasma source commands maybe processed as an embedded input-output controller sequence (IOCsequence) such that the commands are sent as a packet to the IOC andprocessed in sequence with time control accuracy on the order of +/−10ms. This tight control of the fluorine dose during wafer precleanassures that the preclean is adequate to remove native oxides and othercontaminants from the wafer surface, but not so much that it consumessensitive materials from the semiconductor wafer surface (such as metalsilicide or silicon source-drain contacts, polysilicon contacts, orhigh-K gate or capacitor dielectrics).

In cases where a fluorine based wafer preclean results in a fluorinesaturated semiconductor wafer surface after the clean, variouspost-clean wafer treatments may be used to scavenge or otherwise removethe fluorine and thereby promote efficient and uniform subsequent PNL-WNgrowth. Examples of post-clean passivation strategies include thefollowing:

-   -   1. Exposure of the wafer surface to atomic and/or molecular        hydrogen. The atomic hydrogen may be generated using the same        ICP source as described in the preclean description of the        preferred implementation above or by another means. The atomic        or molecular hydrogen many be delivered with an Ar or N₂ carrier        gas for example.    -   2. Exposure of the wafer surface to direct Ar or hydrogen ion        bombardment to strip off surface fluorine and fluorides.    -   3. Exposure of the wafer surface to WF₆ prior to exposure to        B₂H₆ or other reducing agents. The WF₆ may substitute for        surface fluorine and help promote nucleation on the cleaned        surface.    -   4. Exposure of the surface to WF₆ and B₂H₆ simultaneously to for        CVD-W nuclei.    -   5. Exposure of the wafer surface to NH₃.    -   6. A combination of numbers 1 and 5 above. Atomic hydrogen        exposure converts surface fluorine to HF and NH₃ will then        generate NH₄F. Note that NH₄F volatilizes at temperatures        greater than approximately 100 C. A preferred implementation of        this process sequence includes wafer heating to about 250 C or        more during wafer preheat, preclean, and H₂/NH₃ passivation.

In some PNL applications, it may be desirable to provide afour-operation cycle. Here a fourth operation would be reserved forintroducing a dopant into the tungsten nitride. So in addition to thesteps of exposing the substrate to reducing agent, tungsten precursorand nitriding agent, the process also includes a step of exposing thesubstrate to a source of dopant. Examples of such dopant sources includephosphine and arsine. It may be appropriate to introduce theseseparately, rather than together with reducing agent or nitriding agent,because they can be incompatible or possibly explosive in combinationwith other reactants.

Doping is appropriate to change the properties of the tungsten nitride,particularly its work function. The work function is an electronicproperty that affects charge distribution in an adjacent layer.Therefore, doping may be pertinent in applications of the presentinvention relating to creating of a gate electrode or a capacitorelectrode. For other applications such as plug fill and barrier layerformation, doping is not a significant concern. In these applications,the barrier property, conformality, thickness, conductivity andadhesiveness of the tungsten nitride layers are most important.

Note that if a tungsten-tungsten nitride bilayer is formed, the metallictungsten component of this bi-layer can be deposited by either a cyclicPNL process or a bulk CVD process, or some other process if appropriate.CVD has the advantage of producing a lower resistivity film, while PNLhas the advantage of producing a more conformal layer without overhang.

Understand that the invention is not limited to PNL processes employingdiborane or other boron-containing material. More generally, anysuitable reducing agent may be employed. In some cases, the order PNLoperations can be varied so that one does not necessarily start withexposure to the reducing agent. In one alternative, for example, thefirst PNL operation is tungsten precursor absorption. This is followedby reducing agent contact and by nitriding agent contact. Further, thereducing agent and nitriding agent may be provided together in a singleoperation. Still further, exposure to the reducing agent, the tungstenprecursor, and the nitriding agent can be staggered but overlapping intime.

There are many applications of this PNL-WN technology. A few will bedescribed herein. Further, many different process apparatus may beemployed. These include both multi-station and single station depositionchambers. When a single station deposition chamber is employed,different precursor gases are provided from the same chamber plumbing.When a multi-station chamber is employed, the apparatus may move thesubstrate from deposition station to deposition station, with eachchamber dedicated to providing a different single reactant. Further,some deposition chambers can be employed for PNL reactions only, whileothers can be employed for tungsten CVD or other deposition reactions.

Process Flow

A general process flow for the formation of tungsten nitride employed inaccordance with this invention is illustrated in the flowchart ofFIG. 1. First a substrate surface is provided as indicated at 101. Formany embodiments of the invention, the substrate is a semiconductorwafer containing partially fabricated integrated circuitry. It maycomprise trenches and/or vias for interconnects or metal lines, as in aDamascene process. Alternatively, it may comprise thin dielectric layersthat serve as gate or capacitor dielectrics.

On the substrate surface provided at 101, the process forms a tungstenmetal layer. This can be accomplished using one of two methods. In thefirst method, a reducing agent is initially introduced to the substratesurface (block 103) so that the reducing agent (or a moiety on thereducing agent) is adsorbed or otherwise maintained on the substratesurface. See block 105. As indicated above, some embodiments produce a“sacrificial layer,” of reducing agent, particularly when the depositedlayer is a boron layer.

More generally, the reducing agent can be any process-compatiblecompound capable of effectively reducing a tungsten precursor to producea layer of metallic tungsten. Examples of suitable reducing agentsinclude various boron-containing reducing agents, preferably boranessuch as borane (BH3), diborane (B₂H₆), triborane, etc. Examples of otherboron-containing reducing agents include boron halides (e.g. BF₃, BCl₃)with hydrogen. Other reducing agents include silicon hydrides such assilane and organic derivatives thereof.

After the layer of reducing agent has formed on the substrate surface, atungsten-containing precursor gas is introduced to the substratesurface. See block 107. The tungsten-containing precursor is reducedwhen it comes in contact with the adsorbed sacrificial layer on thesubstrate surface, forming a tungsten metal layer. Any suitabletungsten-containing precursor that can be reduced by the reducing agentsacrificial layer to produce a layer of tungsten metal in accordancewith the invention can be used. Examples of suitable tungsten-containingprecursors include WF₆, WCl₆, W(CO)₆, and combinations of these. WF₆ hasbeen found to work particularly well with boron sacrificial layers.Various other tungsten precursors known to those of skill in the art maybe used. Some of these are identified in R. G. Gordon, J. Barton, andSeigi Suh in Materials, Technologies, and Reliability for AdvancedInterconnects and Low-K Dielectrics II, edited by S. Lahiri, (Mat. Res.Soc. Proc. 714E, Pittsburgh Pa., 2001), which is incorporated herein byreference for all purposes.

In an alternative method of forming a metallic tungsten layer, thetungsten-containing precursor is introduced to the substrate surfacebefore the reducing agent. See block 111. The tungsten-containingprecursor can adsorb onto the substrate surface to form a thin layer oftungsten-containing precursor (or tungsten-containing precursor moiety).See block 113. This process sequence variation may be particularlyeffective following a fluorine-based wafer preclean.

Suitable tungsten-containing precursors are those can saturate thesubstrate surface, as by adsorption, and be reduced by a reducing agentto produce a layer of tungsten metal. Generally, the precursorsmentioned above (WF₆, WCl₆, W(CO)₆, and combinations) work well.

After the tungsten-containing precursor is provided on the substratesurface, a reducing agent is introduced to the wafer surface. Thisreduces the adsorbed tungsten-containing precursor layer (block 115),forming a tungsten metal layer. Again, any suitable reducing agent thatcan reduce the adsorbed tungsten-containing precursor layer to produce alayer of tungsten metal in accordance with the invention can be used.Examples include the boron-containing reducing agents, preferablyboranes, more preferably diborane (B₂H₆). Other examples include silanesand derivatives of silanes.

Regardless of whether the process employs operations 111, 113 and 115 or103, 105, and 107, the resulting product is essentially the same: atungsten layer. At this point, as depicted in FIG. 1, the process flowsconverge to a single set of operations.

As shown in block 117, an optional hydrogen plasma treatment may beperformed to remove the excess halogen byproducts and unreacted halogenreactants. This hydrogen plasma treatment can chemically remove thesecompounds that are sometimes adsorbed to the surfaces of the substrateand/or reactor walls. They may result from wafer preclean or tungstenprecursor dose steps.

Once the tungsten metal layer has been formed and optionallyplasma-treated, a nitrogen-containing gas is introduced to the substratesurface to convert at least an upper portion of the tungsten metal totungsten nitride. See block 119. Examples, of suitablenitrogen-containing gases include N₂, NH₃, NF₃, and N₂H₄. At most, thisoperation completely converts the tungsten layer to tungsten nitride. Ifthe resulting nitride layer is not sufficiently thick for the intendedapplication, the steps described above for tungsten deposition andnitridation can be repeated. If the tungsten nitride is of sufficientthickness, the process is complete. See decision 121.

Certain variations on the above procedure will be apparent. First, thesubstrate may be pretreated by an anneal or a wafer preclean by sputteretch (Ar or Ar—H₂) or a reactive clean (F, F₂, NF₃, CF₄, etc). Thepreclean may be used to remove native oxides and other contaminants fromelectrical contacts, or to remove etch residues from contacts or vias.In some situations, it may be possible to enhance PNL-WN adhesion todielectric substrates by preconditioning them with a light sputter-etchor a reactive fluorine-based etch.

Also, the PNL cycle (111, 113, and 115 or 103, 105, and 107) may beinclude a dopant introduction operation. A dopant precursor may beincluded with one of the other three reactants—or alternatively withoutanother reactant. In the alternate case, the PNL cycle will include aseparate operation for dopant introduction, so that the overall cyclenow comprises four separate operations. Examples of dopants includesilicon (from silane for example), phosphorus, arsenic, antimony,bismuth, boron, aluminum, gallium, indium, nitrogen, thallium, andcombinations thereof.

Finally, it may be desirable to introduce a purge gas in between thereactant contact operations; e.g., between operations 103 and 105,and/or between operations 105 and 107, and/or between operations 107 and103. Further, the purge gas can be flowed continuously during any ofthese operations and during a period or periods between the operations.Purge gases are generally inert. Examples include the noble gases (e.g.,argon) and nitrogen.

Hydrogen gas may also be used as a purge or carrier gas. Hydrogen may beused to effectively neutralize residual fluorine from WF₆ and to reducethe levels of fluorine or other halogens in the final PNL-WN film.Hydrogen may also be a required co-reactant with some of the dopantprecursors described in the previous paragraph.

Deposition Unit Operations

As previously mentioned, methods of the present invention may employpulsed nucleation layer (PNL) and related deposition techniques. Thesetechniques are used to form tungsten nitride. The following describes atungsten nitride deposition technique, in accordance with the presentinvention.

PNL deposition, in general, is a method of sequentially depositing aplurality of atomic-scale layers on a wafer surface by sequentiallyinjecting and removing reactants into and from a chamber. PNL depositiondiffers from traditional CVD techniques in that the chemical reactantgases are individually injected, sometimes in the form of a pulse,instead of simultaneously injecting reactant gases, so they are nottypically mixed in the chamber. For example, in a case of using gases Aand B, gas A is first injected into a chamber and the molecules of gas Aare chemically or physically adsorbed to the surface of a substrate,thereby forming a saturated layer of A. Typically, the gas A remainingin the chamber is purged using an inert gas. Thereafter, the gas B isinjected so that it comes in contact with the adsorbed layer of A andreacts to form a reaction product of A and B. Because the reaction islimited by the amount of adsorbed A, and A is relatively evenlydistributed over the substrate surface, excellent step coverage isobtained. B is flowed over the substrate for a period of time sufficientto allow the reaction between A and B to go to completion; i.e., all ofthe adsorbed A is consumed in the reaction. After that point, residualgas B and any byproducts of the reaction are purged from the chamber.The process is repeated for multiple layers of material to be deposited.

PNL deposition is similar to atomic layer deposition (ALD), which alsoinvolves the sequential injection of reactants to a wafer surface. Thepresent invention preferably employs PNL techniques. Generally, however,any ALD or atomic layer epitaxy (ALE) technique may be employed. Somevariations of chemical vapor deposition (CVD) may also be used. Both ALDand PNL can use long or short reagent exposure dose times. The resultingmicrostructure is generally a function of material purity, which isagain tunable for both ALD and PNL. While definitions vary slightly andthere is certainly considerable overlap between ALD and PNL, one keydifference for PNL-W vs ALD-W is that some preferred PNL-W processestypically deposit 3-4 atomic layers of tungsten per complete depositioncycle compared to a single atomic layer (or less) for a moreacademically inclined ALD process. An operational difference is thatpure ALD is typically operated at significantly lower pressures than PNL(mTorr vs 1-100 Torr). Pure ALD may also employ a low-pressure reagentpumping during the reagent purge step rather than relying on inertcarrier purge only to sweep leftover reagent from the depositionchamber. Finally, pure ALD generally cannot include reactive carriergasses such as H₂ during the dosing process, as is done with PNL-W andas is planned for PNL-WN. In the specific case of boron-based PNL-WN,B₂H₆ deposition (unlike SiH₄ or WF₆ adsorption) is not a self-limitingprocess, but rather a real CVD process.

As indicated, tungsten nitride PNL deposition employs a reducing agent,a tungsten precursor, and a nitriding agent, introduced to the wafersurface at different times. The nitriding agent is typically introducedafter an elemental tungsten layer has been formed. This layer may beformed by contacting a reducing agent with an adsorbed tungstenprecursor or by contacting a tungsten precursor with a sacrificial layeror an adsorbed layer of reducing agent. Either way, chamber-purgingoperations separate the individual reactant pulses.

One example of a tungsten nitride PNL deposition technique isillustrated in FIGS. 2A-2D. These figures show sequentialcross-sectional views of the formation of a tungsten nitride layer on asubstrate on a molecular level. Note that these illustrations arecartoon representations to facilitate understanding of PNL deposition.They are shown schematically and are not meant to limit the scope of theinvention.

Referring first to FIG. 2A, a substrate 201 is exposed to diborane 203.The diborane species is adsorbed onto the substrate surface in the formof BH_(x) species 205. These BH_(x) species 205 interact with thesubstrate surface to form elemental boron species 207. Boron was foundto be a particularly good reducing agent for generating tungsten nitrideby PNL. Possibly, it interacts with hydroxyl groups on the dielectricsurface and strips them off to promote adhesion and nucleation. Also,boron may scavenge or otherwise remove residual fluorine from otherprocesses or low-k dielectrics. Presumably, it forms anchor points andthereby promotes good adhesion of the tungsten to the underlyingdielectric layer.

At temperatures greater than about 250 degrees C., the borane speciesrapidly decompose to form elemental boron. This boron generation processcontinues as long as there is boron precursor present and a drivingforce for the decomposition reaction. The reaction proceeds well ondielectric such as silicon dioxide and low k dielectrics, for example.As indicated, the resulting elemental boron layer is sometimes called a“sacrificial layer.”

The formation of the boron sacrificial layer is not a self-limitingreaction. The boron can accumulate to thicknesses well in excess of oneatomic layer. Therefore, exposure to diborane 203 should be limited to apoint where the desired thickness of boron is achieved. Typically,desired thickness is about one to two monolayers of boron species.Preferably, the thickness is less than about 10 angstroms, morepreferably between about 3 and 10 angstroms.

During the formation of a boron sacrificial layer, suitable diborane gaspressures range from about 0.1 and 300 Torr, more preferably betweenabout 1 and 100 Torr, and most preferably between about 1 and 40 Torr.This may apply for reducing agents other than diborane, as well.Preferably, the substrate temperature is at least about 300 degrees C.More preferably, it ranges from about 300 and 500 C, and even morepreferably between about 350 and 400 C. In a specific example, diboraneis provided at a flow rate of about 200 sccm (per station) for a periodbetween about 0.5 and 3 seconds. A 2 second purge is then applied, forexample, prior to introduction of tungsten precursor.

Referring to FIG. 2B, once the boron sacrificial layer 211 is formed tothe desired thickness, the substrate surface 201 is exposed to tungstenhexafluoride (WF₆) gas 209 or other suitable precursor. The tungstenhexafluoride 209 species are reduced when they come in contact with theboron sacrificial layer, thereby forming a thin tungsten metal layer.See layer 215 of FIG. 2C. The thickness of this tungsten metal layer islimited to the amount that can be produced by reaction of the boronsacrificial layer. Note that it may be desirable to limit the amount ofWF₆ to prevent too much exposure to this fluorinated agent, as thefluorine can attack exposed silicon oxides or other features of thesubstrate. Hence, the boron thickness is limited as indicated above. Aninert gas such as argon can be used to dilute the byproducts and removespecies that are already in mobile gas phase in the reactor.

As stated previously, in some embodiments, the surface of the newlyformed tungsten metal layer may also be treated with hydrogen plasma tohelp remove chemical byproducts. The length of time for exposure tohydrogen plasma is sufficient to remove enough halogen byproduct andunreacted precursor that they will not react with or interfere with thereactants introduced in the next deposition cycle. If a hydrogen plasmatreatment is used, it is preferred that the hydrogen plasma pulses arebetween about 0.1 and 30 seconds in duration.

During the reduction reaction of the tungsten-containing precursor withthe boron sacrificial layer, suitable gas pressures are as set forthabove for depositing the reducing agent. Suitable process temperaturesrange from about 200 and 450 C, more preferably between about 250 and350 C. In a specific embodiment, tungsten hexafluoride is provided at aflow rate of about 150 sccm (per station) for a period of between about0.25 and 3 seconds. This is followed by an inert gas purge of about 2seconds, for example.

Referring again to FIG. 2C, once the tungsten metal layer is formed andsufficiently cleaned of byproducts, a nitriding agent in the form of NH₃213 is introduced to the substrate surface. In this example, thetungsten metal layer is approximately one monolayer thick, resulting ina tungsten nitride that is also approximately one monolayer thick. Thetungsten nitride is shown as layer 217 in FIG. 2D. If the tungsten metallayer is more than one monolayer thick, the reaction may be controlledsuch that only the upper portion of the tungsten metal is converted totungsten nitride. In some embodiments, it is preferred that the tungstenbe fully converted to tungsten nitride, as tungsten nitride adheres todielectric much more strongly than elemental tungsten adheres todielectric. This provides another reason to limit the thickness oftungsten formed during the B₂H₆—WF₆ reaction. As stated previously, if athicker layer of tungsten nitride is desired, the steps shown in FIGS.2A-2D can be repeated until the desired thickness is achieved.

During the nitridation reaction, suitable gas pressures are as describedabove for the reducing agent. Suitable process temperatures range fromabout 200 and 450 C, more preferably between about 300 and 400 C. Thenitrogen containing gas may be converted, at least partially, to aplasma, which is, in one embodiment, directed to the substrate bybiasing the substrate with an RF powered electrode. Further, the processmay employ an ultraviolet radiation source or other energizing stimulusto facilitate the nitriding reaction. In a specific embodiment, ammoniais provided at a flow rate of about 250 sccm (per station) for a periodof between about 1 and 10 seconds. This is followed by a purge of about2 seconds, for example.

Not shown in FIGS. 2A-2D is a dopant deposition unit operation. Suchoperation is optional, depending upon the application of the tungstennitride layer. If a separate dopant deposition operation is employed, itmay employ a pressure matching one of those set forth above for thereducing agent. In addition, it may employ a substrate temperaturesuitable for driving the reaction that converts and dopant precursor toelemental dopant. The flow rate and exposure time are chosen to meet thelevel of doping required in the final product. Examples of precursorcompounds include B₂H₆, NH₃, SiH₄, phosphine and arsine.

As indicated, one aspect of the current invention provides inclusion ofa thin layer of tungsten deposited by PNL or other means prior to thefirst deposition of PNL-WN. In one implementation, this is accomplishedby simply suppressing the nitridation sequence of the PNL-WN process forthe first few deposition cycles. After the W seed layer is deposited,the PNL-WN process continues without vacuum break to deposit the desiredtotal amount of PNL-WN and possibly PNL-W nucleation and CVD-W.

The inventors have observed that they can achieve a significantenhancement in adhesion on dielectrics by introducing a thin (<50 Å andpreferably about 10-50 Å) tungsten seed layer prior to deposition ofPNL-WN. Possible mechanisms for this improvement include but are notlimited to a change in the stoichiometry of the WN film at theWN-dielectric interface to a more perfect W2N composition if theintrinsic PNL-WN process is nitrogen-rich. X-ray diffraction studieshave shown that films with more pronounced W2N microstructure havebetter adhesion.

The thin tungsten seed may also be available for reaction with the metalsilicide or silicon at the base of a contact, bitline, or otherstructure. The resulting tungsten silicide is able to reduce theelectrical resistance of a WN-silicide (NiSix, CoSix, TiSix) or WN—Si(N+, P+, poly-Si) interface much like titanium does in current tungstenmetallization schemes. The tungsten seed can react with silicon to formsilicide at temperatures greater than about 500 C (preferably betweenabout 550 and 650 C). Thus, this thin tungsten silicide interfaciallayer can be of great benefit in reducing contact resistance of tungstenand tungsten nitride contacts to silicon or metal silicides when thetraditional Ti—TiN—W film stack has been replaced by aW-seed/PNL-WN/PNL-W/CVD-W film stack. The thin tungsten seed is readilyconverted to tungsten silicide at temperatures greater than about 550 C,which can dramatically lower the resistance of the WN—Si interface.(PNL-WN, on the other hand, is resistant to reaction with silicon untilreaching temperatures greater than approximately 750 C.) The tungstenseed embodiment enables the formation of a thin and readily controllabletungsten silicide layer for reduced contact resistance.

In another approach, the invention provides alternating layers of PNL-WNand tungsten. When the layers are very thin (preferably less than about10 Å each) the layered structure can self-anneal into a moretungsten-rich form of tungsten nitride. By utilizing the widestoichiometry control offered by layered WN—W deposition it is possibleto modulate the work function of the composite material. This is veryimportant for both gate electrode and capacitor electrode applications.It may also be beneficial in improving the thermal stability of thefinal film by chemically binding excess nitrogen into a stable tungstennitride stoichiometry such as W2N.

It may also be desirable to create layered structures beginning witheither PNL-WN or PNL-W. In a preferred embodiment, the layered structurebegins and ends with tungsten.

It may be advantageous to perform the PNL-WN deposition in ahydrogen-argon ambient or other fluorine scavenging environment. It isimportant to the success of many applications to create a PNL-WN filmwith very low fluorine content. One strategy by which this objective canbe achieved is to use an Ar—H₂ carrier gas mixture. The H₂ reacts withfree fluorine to generate HF that can be effectively pumped away for thesemiconductor wafer. In one preferred embodiment the Ar—H₂ carrier gasmixture is roughly 50% hydrogen.

Applications

Below are descriptions of various exemplary applications of the presentinvention. Note that these descriptions are presented only as examples.They are not meant to exclude other applications of the invention. Norare they meant to exclude the use of the invention from variations orcombinations of methods described.

Application 1: Copper Diffusion Barrier

Damascene fabrication processes may employ tungsten nitrides as a copperdiffusion barrier. It has also been found that the tungsten nitridelayer serves a second function: as an adhesion layer for the coppermetallization layer.

The copper metallization layer may comprise two components: a first thinseed layer and a second electrolytic bulk copper layer. The seed layermay be deposited by physical vapor deposition (PVD), CVD, ALD, orelectroless deposition, for example. Electroless deposition may beaccomplished with appropriate plating solutions, including a source ofcopper ions, a reducing agent, and in some cases a base to render thesolution alkaline. Preferably the PVD process is performed with a hollowcathode magnetron, but other PVD apparatus may be employed as well.

The tungsten nitride is deposited by PNL or ALD as described above. Thecopper seed layer may be formed directly on the nitride or indirectlyvia a metallic tungsten layer. If a metallic tungsten layer is employed,a stack comprised of dielectric, tungsten nitride, metallic tungsten,and copper results. The metallic tungsten may be formed by PNL, ALD, CVDor a combination of these processes.

The following list presents numerous process options.

1. PNL WNx-PNLW-CVDW (two step tungsten deposition)-HCM-Cu seed(HCM=hollow cathode magnetron)—followed by electrolytic copper plating

2. PNL WNx-PNLW-HCM-Cu seed—followed by electrolytic copper plating

3. PNL WNx-CVDW-HCM-Cu seed—followed by electrolytic copper plating

4. PNL WNx-HCM-Cu seed—followed by electrolytic copper plating

5. PNL WNx-PNLW-CVDW—followed by electroless copper seed andelectrolytic Cu fill

6. PNL WNx-PNLW followed by electroless copper seed and electrolytic Cufill

7. PNL WNx followed by electroless copper seed and electrolytic Cu fill

In each case, PNL-WNx serves as an adhesion layer. PNL-WNx and/or PNL-Wcan both serve as copper barriers. Advantages of WN—W barriers overconventional barriers such as Ta—TaN include conformality (100% in >10:1AR contacts) and improved electromigration resistance due to improvedtungsten-copper bonding.

For integration schemes in which WN—W is not integrated with copper seeddeposition (in a single vacuum integrated tool), native oxide can beremoved from the W surface by argon or argon-hydrogen ion bombardment.After cleaning, W—Cu adhesion is dramatically improved for non-vacuumintegrated W—Cu.

Note that options 1 and 5 include a two-step tungsten depositionprocess, including both CVD and PNL deposition processes. This may bedone because CVD deposited films generally have a low resistivity, whilePNL deposited films have good step coverage.

Note that for options 5-7 above, a PNL-WN/PNL-W stack is the preferredimplementation because direct plating is more favorable on a tungstensurface than on tungsten nitride. Direct plating on a tungsten/tungstennitride barrier stack is also most preferably carried out in a basicplating solution to facilitate the removal of native tungsten oxide fromthe Cu—W interface by solvation of tungsten oxide.

Application 2: Tungsten Plugfill

The PNL tungsten nitride deposition method of this invention can also beused in processes for generating tungsten plugs for contact or via fillin IC wafer fabrication. The tungsten nitride layer serves as adiffusion barrier and adhesion layer for the tungsten contact. The PNLWN barrier and/or adhesion layer for direct tungsten plugfill contactstungsten, metal silicides (such as TiSi_(x), CoSi_(x), NiSi_(x), orWSi_(x)), silicon (N+ or P+), or other electrically conductivematerials. It may also be preferably combined in an integrated contactplug film stack including PNL-W (seed layer)/PNL-WN (barrier-adhesionlayer)/PNL-W (nucleation layer)/CVD-W (primary conductor and bulkplugfill). This proposed tungsten plugfill integration scheme isintended to replace the traditional Ti/TiN/W tungsten plugfill filmstack.

In current semiconductor metallization schemes for 90-nm and higherdevice geometries, tungsten is used as a primary conductor for manycontact and via applications. The typical film stack in use today isPVD-Ti/CVD-TiN/W nucleation/CVD-W. With PNL-WN it is possible tosimplify this film stack to PNL-WN/PNL-W (nucleation)/CVD-W. Thisapplication of PNL-WN has many advantages over current practices in theindustry. First, by eliminating the need for Ti—TiN semiconductormanufacturers are able to eliminate an entire tool set, greatlysimplifying the process flow and reducing the cost to manufacturesemiconductor devices. The process temperatures for PNL-WN and PNL-W are(approximately) <400 C and <300 C in a typical implementation, comparedto >500 C for CVD TiN from TiCl₄ or >450 C for CVD-TiN from TDMAT. Thisreduction in processing temperature allows advanced manufacturers oflogic and memory devices to move from titanium and cobalt silicide metalcontacts to nickel silicide, which undergoes a phase change to a newphase with dramatically higher electrical resistance at temperaturesabove 450 C. Elimination of PVD-Ti in the tungsten metallization stackeliminates the overhang associated with all PVD processes at the mouthof a contact or via. This creates an intrinsically re-entrant featurewhich leads to center seam formation during subsequent filling of thecylindrical contacts or vias with CVD-W. By eliminating the overhang byreplacing Ti—TiN with PNL-WN, no overhang is created at the featureopening prior to CVD-W and the resulting center seam is reduced. Suchseams can be opened during CMP of the tungsten film to complete thetungsten metallization. Finally, by reducing the total thickness ofliner barrier required inside a contact or via prior to CVD-W, PNL-WNimplementation enables a reduction in contact or via resistance comparedto the traditional PVD-Ti/CVD-TiN/CVD-W film stack. This reduction inresistance comes about because a larger percentage of the contact orvias cross section is available for deposition of low resistivitytungsten rather than higher resistivity Ti and TiN.

To recap, some advantages of the tungsten nitride plugfill integrationscheme over the traditional Ti—TiN integration scheme are as follows:

-   1) The ability of the PNL-WN/PNL-W processes to fill contacts and    vias with aspect ratios greater than 20:1. This is a significant    advance over traditional CVD-TiN and PVD-Ti step coverage    performance-   2) Elimination of Ti—TiN deposition equipment and processing steps.    This simplification will substantially reduce the manufacturing cost    of the tungsten plugfill process. In a preferred implementation    wafer preclean, PNL-WN, PNL-W, and CVD-W are all completed in a    single wafer pass through an integrated cluster tool. This saves on    wafer moves from tool to tool, semiconductor cleanroom floor space    requirements, and semiconductor manufacturing capital equipment cost    by eliminating the Ti—TiN deposition tool.-   3) Reduction in semiconductor wafer maximum processing temperature    requirements. Modern semiconductor wafer processing thermal budget    requirements call for maximum processing temperatures <450 C for    contact metallization and <350 C for vias in low-K dielectrics.    PNL-WN and PNL-W are both typically deposited with wafer    temperatures <300 C in the preferred implementation.-   4) Reduction in post-CMP center seam opening (or coring) of tungsten    plugs. As tungsten grows from the sides of a contact or via to the    center, a thin seam is typically left at the centerline of the plug.    In the case of a Ti—TiN liner barrier stack, the relatively poor    step coverage of (PVD) Ti and (CVD) TiN result in an overhang    feature at the mouth of the contact. This can result in a feature    with a smaller diameter at the top than in the mid-section of the    plug. Such an overhang inevitably produces an open seam inside the    plug because the growing tungsten film seals the top of the feature    before the midsection of the feature is completely filled. Such    seams can be exposed during CMP and result in wafer defects. In the    case of a PNL-WN/PNL-W liner-barrier film stack, both materials have    virtually 100% step coverage and can be deposited very thinly (<50    Angstroms), which results in no overhang and no resulting seam    during CVD plugfill.-   5) By reducing the total thickness of the liner barrier layer from,    say, 200 Angstroms for a typical Ti—TiN implementation to <50    Angstroms for PNL-WN (in the preferred implementation), it is    possible to fill the contact or via with a greater amount of CVD-W,    which is the primary conductor and significantly lower in    resistivity than PVD-Ti, CVD-TiN, or PNL-WN. This can reduce the    electrical resistance of the contact or via for small diameter    contacts where the liner barrier film thickness is an appreciable    percentage of the feature radius.

In this invention, PNL tungsten nitride can be formed in the vias orcontact holes directly on the dielectric or with a PNL-W seed layer. Thetungsten may be deposited by PNL, ALD, CVD, or a combination of these.Also, the process may be integrated with a degas and/or precleanoperation (e.g., a plasma etch) prior to the PNL tungsten nitridedeposition. In some cases it may be advantageous to create a combinedTiN/WN barrier layer.

The reactor employed for this application may support single-waferprocessing or multi-station sequential deposition, with tungsten nitridePNL and tungsten CVD integrated in a single module. In the preferredimplementation wafer pre-heat, preclean, and PNL-WN deposition arecombined in a one multi-station process module and a second processmodule is dedicated to the deposition of PNL-W and CVD-W. In somesituations wafer preheat/degas, and wafer preclean may each be givenindependent process modules on an integrated cluster tool to providegreater process flexibility.

Exemplary process flows include the following:

-   -   1. PNL WN-PNLW nucleation layer-CVD-W plugfill    -   2. PNL-W seed/PNL-WN barrier adhesion/PNL-W nucleation/CVD-W        bulk plugfill    -   3. preheat/wafer preclean/PNL-W seed/PNL-WN        barrier-adhesion/PNL-W nucleation/CVD-W plugfill    -   4. PNL WN-CVD-W plugfill    -   5. WN—W integrated with degas, preclean (DFE or reactive clean)    -   6. WN—W integrated with degas, preclean (DFE or reactive clean)        and HCM-Ti (a thin titanium layer deposited via a hollow cathode        magnetron).

Note that “DFE” is Dual Frequency Etch. As an example, Novellus' INOVAwafer preclean (Novellus Systems, Inc., San Jose, Calif.) uses Ar ionsfrom a dual frequency inductive plasma to provide high plasma density(high frequency component) and independently controllable ionacceleration (low frequency component).

Application 3: Capacitor Electrode

Current DRAM capacitor electrodes making use of TiN/polysiliconelectrodes suffer from the high deposition temperatures of both CVD-TiN(>500° C. from TiCl₄) and polysilicon CVD (>600° C. from SiH₄). Thesehigh deposition temperatures drive reaction byproducts into thecapacitor dielectric and in so doing reduce the dielectric constant ofthe dielectric and the capacitance of the resulting storage capacitor.High step coverage is an absolute requirement in advanced memory cellelectrode formation, and the near 100% step coverage of PNL-WN infeatures with aspect ratios >15:1 is far superior to previous CVD andPVD tungsten nitride processes. Based on literature reports, the workfunctions of WN and W are in the 4 to 4.5 eV range, which issignificantly higher than typical CVD-TiN work functions (2.2 eV). Highelectrode work functions are known to reduce leakage in memory cellcapacitors.

In this application, the PNL deposited tungsten nitride layer is used asa metal electrode either alone or in a PNL-WN/PNL-W film stack. Moregenerally, the tungsten nitride layer can function as an adhesion layer,barrier layer, and/or as a primary electrical conductor for a top orbottom capacitor electrode. Again, the tungsten may be deposited by PNL,ALD, CVD, or a combination thereof. Degas and/or preclean may beemployed. And single wafer processing or multi-station sequentialdeposition may be employed.

Note that integrated circuit capacitor electrodes are currently madefrom film stack of CVD-TiN and highly doped polysilicon. The depositiontemperatures for TiCl₄-based CVD-TiN and poly silicon are >550 Cand >600 C, respectively. These high temperatures result in the drivingof impurities into the capacitor dielectric (e.g. Cl) and the oxidationof the TiN barrier layer, both of which reduce capacitance and increasecapacitor leakage. A WN—W capacitor electrode can dramatically reducethe manufacturing thermal cycles with resulting leakage reductions andimprovements or post anneal capacitance for comparable leakage. Thefollowing process flows can be used to deposit top or bottom capacitorelectrodes.

-   1) PNL WN/PNLW nucleation layer/CVD-W-   2) PNL-W/PNL-WN/PNL-W/CVD-W-   3) PNL WN-CVD-W plugfill-   4) WN—W integrated with degas, preclean (DFE or reactive clean)

The capacitors may be trench capacitors, fin capacitors, platecapacitors or any other structure suitable for IC applications. In thecase of stacked capacitors the bottom electrode may be deposited on apolysilicon bottom electrode to facilitate structure formation. Theextremely high step coverage of PNL-WN and PNL-W are enabling featuresrequired for implementation of PNL-WN for modern semiconductor memorycell electrodes.

Application 4: Gate Electrode

In this application, tungsten nitride functions as an adhesion layer,barrier layer, or primary conductor in a gate electrode. Tungstennitride may be applied directly on the gate dielectric or on apolysilicon electrode to reduce polysilicon line thickness requirements.

Some of requirements for a transistor gate application include a tunablework function, thermal stability, and resistance to oxidation. Modifyingthe W/N stoichiometry of the as-deposited film, or adding dopants suchas boron (from diborane for example), silicon (from silane for example)and/or nitrogen (from ammonia for example) can tune the work function ofPNL tungsten nitride. In addition to boron and nitrogen, typical III-Vdopant materials such as Al, Ga, P, and As may also be employed. Anothereffective way to modulate the work function of PNL-WN is to generate alayered structure of PNL-WN and PNL-W. The number, thickness, andsequence of the layers can be varied, but the preferred implementationis for very thin (<10 Å) layers of alternating PNL-W and PNL-WN,beginning and ending with PNL-W.

As a gate electrode, a PNL tungsten nitride or atungsten-nitride/tungsten film stack provides a metal gate that resiststhe charge depletion phenomenon commonly observed in non-metallic gateelectrodes such as those fabricated from polysilicon. Charge depletioneffectively increases the gate dielectric thickness. Atungsten-nitride/tungsten gate electrode may also be formed on top of apolysilicon gate electrode to reduce the height requirement of thepolysilicon gate without changing the gate dielectric/polysiliconinterface.

As discussed above, it may be valuable to fabricate layered PNL-W/PNL-WNgate electrode structures to facilitate work function modulations formixed N+ and P+ transistor devices.

Various possible process flow implementations include

-   1) PNL-WN-PNL-W-CVD-W bulk deposition and interconnect-   2) Layered PNL-W/PNL-WN/CVD-W bulk deposition and interconnect-   3) PNL-WN/PNL-W gate electrode with poly-Si plug and interconnect-   4) PNL-WN/PNL-W/CVD-W on a thin poly-Si gate electrode for reduced    poly-Si thickness requirement.-   5) PNL WN-CVD-W plugfill.    Tool configuration options include-   1) WN—W integrated with degas, preclean (DFE or reactive clean) and-   2) single-wafer processing or multi-station sequential deposition    with WN, PNL and WCVD integrated in a single module.

Application 5: Other Uses

In one application, PNL WN serves as a barrier and adhesion layer forthe deposition of bitline or wordline local interconnects in DRAMdevices. In another application it serves as an adhesion layer for Wdeposition on oxide in semiconductor applications such as light shieldfor CCD devices. Still further, PNL WN may serve as a hardmask forpatterning low-K dielectrics. A WN—W stack can be deposited at lowertemperature and with less exposure of photoresist to harmful amines thanconventional PECVD SiN hardmasks. Many other applications will occur tothose of skill in the art.

Deposition Apparatus

The methods of the invention may be carried out in any number of processchambers. Examples include the Novellus Systems Concept 2 Altus chamber,the Novellus Concept 3 Altus processing chamber, or any of a variety ofother commercially available CVD tools. In some cases, the reactorcontains multiple deposition stations in which parallel reactions cantake place. See, e.g., U.S. Pat. No. 6,143,082, which is incorporatedherein by reference for all purposes. Thus, in some embodiments, thepulsed nucleation process is performed at a first station that is one ofmultiple deposition stations provided within a single depositionchamber. Thus, the reducing gases, tungsten-containing precursor gasesand nitridation gases are alternately introduced to the surface of thesemiconductor substrate, at the first station, using an individual gassupply system that creates a localized atmosphere at the substratesurface. Between the successive exposures to reactant gases, the chamberis purged using an inert gas or an inert gas in conjunction withhydrogen. This process may take place in parallel at the multipledeposition stations. Alternatively, or in addition, some stations can bereserved for PNL deposition of tungsten nitride while others arereserved for PNL or CVD formation of tungsten. In such cases, the PNLdeposition of tungsten nitride may take place at one or more stations.After the required PNL cycles are completed to deposit the fullthickness of nitride, the substrate is moved to a different stationwhere metallic tungsten is deposited on the new tungsten nitride layer.In a preferred implementation PNL-WN (including tungsten seed layerformation) is deposited in a dedicated process module and PNL-W andCVD-W are deposited in another process module on a vacuum integratedcluster tool.

In other embodiments, the semiconductor substrate is moved betweendifferent deposition stations during a single PNL deposition cycle. Inthis approach, different stations are dedicated to different processeswithin the cycle. For example, one or two stations might providereducing agent, one or two other stations might provide tungstenprecursor and still other stations might provide nitriding agent. And insome embodiments, certain stations can be reserved for providing dopantprecursors. The various stations can also provide for the simultaneousdelivery of inert gases with the dedicated gases. Overall, tungstennitride is deposited by moving wafers from station to station such thatthe wafer is sequentially exposed to the reducing gases, then thetungsten-containing precursor gases, then nitridation gases, repeatedlyuntil the desired thickness of tungsten is obtained.

In any of these scenarios, the wafers may be indexed from one depositionstation to the next to enable parallel wafer processing. Indexingcontinues until all substrates are coated to the desired thickness. Anynumber of deposition stations, each capable of having a localizedatmosphere isolated from adjacent stations, is possible within thesingle chamber.

As will be appreciated in the art, each such deposition station willtypically have a heated substrate platen for holding and heating asubstrate to a predetermined temperature. In addition, each may have abackside gas distribution system to prevent deposition of the W film onthe backside of the substrate, and a vacuum clamping manifold forclamping the substrate to the platen. Finally, the deposition chambercan be equipped with a transport system for transporting wafers orsubstrates into and out of the chamber as well as between depositionstations.

Another aspect of the invention provides a module containing one or moreof the following design elements for alternating deposition of tungstennitride and/or stacks comprising tungsten nitride and tungsten:

a plurality of deposition stations comprising a showerhead or dispersionplate for uniform gas introduction paired with a heated pedestal thatholds a wafer underneath the showerhead;

a plurality of deposition stations with showerheads flush-mounted withthe top of the module vacuum chamber to minimize gas re-circulation inthe module and promote efficient purging between alternating depositionsteps;

a fully purged top plate of the module vacuum chamber consisting of apurge gas plenum covering the top plate area not occupied by depositionshowerheads wherein improved station-to-station isolation and reducedpurge times between deposition cycles are obtained; or

a vacuum chamber in which the heated pedestals from each depositionstation are completely or partially isolated from each other by anannular pumping ring connected to the chamber exhaust. This featurefurther enhances station-to-station isolation and enables differentprocesses to be run simultaneously on alternate stations in the samemodule.

The module may further comprise a mechanism, provided to eachshowerhead, for creating a RF plasma in between the showerheads and thesubstrate platens. Preferably, such means comprise an RF energy source,a match network, and the necessary electrical connections. In anotherembodiment, the module may further comprise means for creating a remoteplasma in the chamber. Some modules may provide a divert line connecteddirectly to the process vacuum exhaust (pump or vacuum foreline) suchthat process gasses can bypass the deposition chamber during the timeimmediately after their respective mass flow controllers (MFCs) areturned on. In addition, the gas delivery system may be provided with amechanism for controlling the line charge volume. This can be importantin precisely timing delivery of nitriding agent, tungsten precursor(e.g., WF₆), and/or reducing agent (e.g., SiH₄ and B₂H₆). With suchhardware features, all gasses that pulse on and off during PNL can bedelivered with a divert and line charge process sequence.

The invention may be implemented using a gas manifold system, whichprovides line charges to the various gas distribution lines as shownschematically in FIG. 3. Manifold 304 has an input 302 from a source ofthe tungsten-containing precursor gas (not shown), manifold 311 has aninput 309 from a source of diborane or other reducing gas (not shown)and manifold 319 has an input 321 from a source of nitriding agent (notshown). The manifolds, 304, 311 and 321 provide the tungsten-containingprecursor gas, reducing gas and nitriding agent to the depositionchamber through valved distribution lines, 305, 313 and 325respectively. The various valves are opened or closed to provide a linecharge, i.e., to pressurize the distribution lines. For example, topressurize distribution line 305, valve 306 is closed to vacuum andvalve 308 is closed. After a suitable increment of time, valve 308 isopened (valve 315 is closed) and the tungsten-containing precursor gasis delivered to the chamber. After a suitable time for delivery of thegas, valve 308 is closed. The chamber can then be purged to a vacuum byopening of valve 306 to vacuum.

Similar processes are used to deliver the reducing gas and the nitridingagent. To introduce the reducing gas, for example, distribution line 313is charged by closing valve 315 and closing valve 317 to vacuum. Openingof valve 315 allows for delivery of the reducing gas to the chamber.Similarly, to introduce the nitrding agent, distribution line 325 ischarged by closing valve 327 and closing valve 323 to vacuum. Opening ofvalve 327 allows for delivery of the ammonia or other nitriding agent tothe chamber. It has been found that the amount of time allowed for linecharges changes the amount and timing of the initial delivery of thegas. Some examples of suitable line charge times are presented below.

FIG. 3 also shows vacuum pumps in which valves 306, 317 and 323,respectively, can be opened to purge the system. The supply of gasthrough the various distribution lines is controlled by a controller,such as a mass flow controller which is controlled by a microprocessor,a digital signal processor or the like, that is programmed with the flowrates, duration of the flow, and the sequencing of the processes.

If the process employs both a boron-containing reducing agent and asilane reducing agent, there may be two subsystems for the reducingagent: one for the boron-containing reducing agent and another for thesilane.

Note that the PNL processes described above may require precise timingof valves and mass flow controllers (MFCs) supplying pulses of reagentto the semiconductor substrate during PNL-WN deposition. In one way tomake this possible, valve and MFC commands are delivered to embeddeddigital input-output controllers (IOC) in discrete packets ofinformation containing instructions for all time-critical commands forall or a part of a PNL deposition sequence. The C2 and C3 ALTUS systemsof Novellus Systems, Inc. provide at least one IOC sequence. The IOCscan be physically located at various points in the apparatus; e.g.,within the process module or on a stand-alone power rack standing somedistance away from the process module. There are typically multiple IOCsin each module (e.g., 3 per module). With respect to the actualinstructions included in a sequence, all commands for controlling valvesand setting flow for MFCs (for all carrier and reactant gases) may beincluded in a single IOC sequence. This assures that the timing of allthe devices is tightly controlled from an absolute standpoint and alsorelative to each other. There are typically multiple IOC sequencesrunning at any given time. This allows for, say, PNL to run at stations1-2 with all timing controlled for all the hardware components needed todeposit PNL-WN at those stations. A second sequence might be runningconcurrently to deposit CVD-W at other deposition stations in the samemodule. The relative timing of the devices controlling the delivery ofreagents to stations 3-5 is important within that group of devices, butthe relative timing of the PNL process at stations 1-2 can be offsetfrom the relative timing of CVD at stations 3-5. An IOC translates theinformation in a packetized sequence and delivers digital or analogcommand signals directly to MFC or pneumatic solenoid banks controllingthe valves. This implementation reduces delays in command execution atthe valve or MFC to as little as 5 ms. Typical control systems in whichcommands are issued one by one to the IOC are subject to communicationdelays between the computer controlling module operation and the IOC.Delays in the single-command implementation can exceed 250 ms.

In one example, to achieve good response and repeatability, thenitriding agent flow may be introduced by first enabling flow through anitriding agent Mass Flow Controller (MFC) and diverting the flow to aprocess vacuum pump to stabilize flow before introducing the agent intothe deposition chamber. To stabilize the nitriding agent flow, theoutlet valve 327 is closed while divert valve 323 is open. The manifoldsystem then pressurizes delivery line 325 to assure a controlled initialburst of nitriding agent by closing the divert valve 323 with theprocess outlet valve 327 closed for between about 0.10 and 3.00 seconds.Next, the system opens the outlet valve 327 to the deposition chamberwith the divert valve closed to deliver nitriding agent to the processchamber during deposition. Preferably, all valve timing is controlledusing an embedded input-output controller command sequence as describedabove. The above process may be applied to deposition of tungstennucleation layers, bulk layers, and/or cap layers, using PNL or CVD.

One manifold system sequence for delivering a boron-containing gas(e.g., diborane) to the chamber involves the following operations.First, the system divert a diborane-carrier gas mixture to a vacuum pumpfor a period of time while the MFC or other flow controlling devicestabilizes. Preferably, this is done for between about 0.5 and 5 secondsusing a mixture of 5% by volume diborane in a nitrogen carrier gas. Nextthe system pressurizes the diborane delivery manifold by closing boththe divert valve and the outlet to the deposition chamber. In oneimplementation, this is done for a period of time between about 0.1 to 5seconds. This creates an initial burst of reagent when the outlet to thedeposition chamber is opened. In one implementation, the outlet valve isopened for a period of between about 0.1 to 10 seconds. This is thenfollowed by purging diborane from the chamber using a suitable carriergas.

A pulse of tungsten-containing gas may be generated as follows.Initially, the system diverts WF₆ (an example of the gas) to a vacuumpump for a period of time while the MFC or other flow-controlling devicestabilizes. This may be done for a period of between about 0.5 to 5seconds in one example. Next, the system pressurizes the tungsten gasdelivery manifold by closing both the divert outlet 306 and the outlet308 to the deposition chamber. This may be done for a period of betweenabout 0.1 and 5 seconds, for example, to create an initial burst ofreagent when the outlet to the deposition chamber is opened. This isaccomplished by opening outlet valve 308 for between about 0.1 and 10seconds in one example. Thereafter, the tungsten-containing gas ispurged from the deposition chamber using a suitable purge gas.

The pulsed flow of silane or other reducing gas may be implemented in asimilar manner by controlling divert valve 317 and outlet valve 315. Thedivert, line pressurization, and flow steps may be timed as presentedabove for the tungsten-containing gas example. After pulsing withreducing gas for a period of between about 0.1 and 10 seconds, outletvalve 315 is closed and a purge gas is flowed through the chamber.

A listing of hardware elements that may be employed in accordance withthe present invention follows. Some of these were identified above.

-   1) A fully heated and insulated PNL-WN process module such that all    internal surfaces are maintained at 100 C or above to avoid    condensation of NH4F, a byproduct of the B2H6-WF6-NH3 reaction.-   2) A system of gas manifolds such that a single reagent MFC (paired    with an inert carrier gas MFC) is split to supply this reagent to    multiple deposition stations. This implementation effectively    reduces hardware cost by sharing components across multiple    deposition stations. It also reduces wafer to wafer and station to    station variability by supplying multiple stations from a single    source-   3) A system with shared reagent manifolds as discussed above (#2) in    which each outlet of the manifold to a deposition station is    individually valved. This enables the user to select whether or not    a given reagent should be delivered to a specific deposition station    during a given deposition cycle. This further enhances the ability    of the tool to deliver layered films with multiple compositions. For    example, the ammonia outlet at the first deposition station may be    closed for one or more deposition cycles to promote the growth of a    thin tungsten seed (from B2H6-WF6 reaction).-   4) Inclusion backside gas delivery hardware to deliver Ar, H2 and    NH3 through heated wafer susceptors to the backside of the wafer.    The presence of backside ammonia facilitates stoichiometry control    across the entire surface of the wafer. Without backside reagent    control one or more reagents may be depleted at the wafer edge.-   5) Inclusion of a wafer preheat station within a PNL-WN deposition    module.-   6) Inclusion of a wafer preclean station within a PNL-WN deposition    module-   7) Inclusion of a reactive wafer preclean module using atomic and/or    molecular fluorine to remove native oxides, etch residues, and other    contaminants from semiconductor wafer surfaces    -   a) Use of an inductively coupled plasma source to dissociate        NF3, CF4, C2F6 or other fluorine containing gas as a source for        atomic and molecular fluorine.    -   b) Using reagent divert and line charge gas handling as        described above to precisely control the dose amount of fluorine        precursor arriving at the wafer for preclean. Over etch may        cause undesirable fluorine attack of delicate structures such as        shallow silicide junctions in direct plugfill applications.    -   8) Inclusion of a total chamber purge such that the majority of        the top of the process module not occupied by reagent        showerheads actively delivering reagent to semiconductor is        actively purged by an inert carrier. In the preferred        implementation the process showerheads are embedded in a purge        plenum nominally flush with the showerhead face. This        configuration can eliminate gas recirculation inside the        deposition chamber and dramatically reduce the time required to        flush reagent from the chamber. The gas curtain can also enhance        station-to-station isolation in a multi-station chamber        architecture.    -   9) Control all time-critical commands to valves and MFC's        through an input-output controller (IOC) such that valve timing        sequences are delivered to the IOC in a packet sufficiently        comprehensive that all the commands for a deposition sequence        can be run without pausing to write or read data to the computer        controlling the deposition module. By eliminating slow write and        read communication the IOC is able to control valve and other        device timing within 10-20 ms, which is required for PNL-WN        processing due to the short line charge, dose, and purge times        required by some sequences.

It is understood that the examples and embodiments described herein arefor illustrative purposes only and that various modifications or changesin light thereof will be suggested to persons skilled in the art and areto be included within the spirit and purview of this application andscope of the appended claims. For example, generally, all references toa hollow cathode magnetron can be replaced with references to ageneral-purpose physical vapor deposition (PVD) reactor. Allpublications, patents, and patent applications cited herein are herebyincorporated by reference in their entirety.

1. A method of forming a tungsten nitride layer on a substrate, themethod comprising: (a) positioning the substrate in a depositionchamber; (b) depositing one or more layers of pulsed deposition tungsten(W) on the substrate; and (c) depositing one or more layers of pulseddeposition tungsten nitride (WN) on the one or more layers of pulseddeposition tungsten to generate a W—WN composite film comprising eithera bilayer of W—WN or a multi-layered structure of multiple tungsten (W)and tungsten nitride (WN) layers, wherein step (c) comprises at leastone pulse deposition cycle wherein the reducing agent comprises boronand at least one pulse deposition cycle wherein the reducing agentcomprises a silicon hydride.
 2. The method of claim 1, wherein the ratioof W and N atoms are present in the W and WN layers in a ratio ofapproximately 2-to-1.
 3. The method of claim 1, further comprisingproviding a dopant to the one or more layers of pulsed depositiontungsten nitride.
 4. The method of claim 3, wherein the dopant is atleast one of phosphorus, arsenic, antimony, bismuth, boron, aluminum,gallium, indium, nitrogen, and thallium.
 5. The method of claim 1 inwhich tungsten nitride deposition is carried out in a multi-stationreaction chamber in which (i) pulsed nucleation layer (PNL) tungsten isdeposited at one or more stations in the chamber; (ii) tungsten nitrideis deposited at one or more deposition stations in the chamber; and(iii) the substrate is moved from one deposition station to another suchthat the layered film of tungsten nitride and PNL-tungsten is formed. 6.The method of claim 1, further comprising forming a metallic tungstenplug on the composite film to form a tungsten interconnect, wherein thecomposite film serves as at least one of an adhesion layer, a barrierlayer, and/or a nucleation layer for subsequent tungsten growth.
 7. Themethod of claim 1 further comprising repeating (b)-(c) to generate amulti-layered structure of multiple tungsten (W) and tungsten nitride(WN) layers.
 8. A method of forming a tungsten nitride layer on asubstrate, the method comprising: (a) positioning the substrate in adeposition chamber; (b) depositing one or more layers of pulseddeposition tungsten (W) on the substrate; and (c) depositing one or morelayers of pulsed deposition tungsten nitride (WN) on the one or morelayers of pulsed deposition tungsten to generate a W—WN composite filmcomprising either a bilayer of W—WN or a multi-layered structure ofmultiple tungsten (W) and tungsten nitride (WN) layers, wherein step (c)comprises: (i) depositing a gas phase boron-containing agent onto theone or more layers of pulsed deposition tungsten to form aboron-containing sacrificial layer on the one or more layers of pulseddeposition tungsten; (ii) exposing the boron-containing sacrificiallayer to a tungsten containing precursor to form a tungsten layer; and(iii) exposing the tungsten layer to a nitriding agent to form at leasta portion of the tungsten nitride layer.
 9. A method of forming a W—WNlayer on a substrate having a surface, the method comprising: (a)positioning the substrate in a deposition chamber; (b) depositingmetallic tungsten on the substrate surface; (c) depositing a gas phaseboron-containing agent onto the metallic tungsten to form aboron-containing sacrificial layer thereon; (d) exposing theboron-containing sacrificial layer to a tungsten containing precursor toform a tungsten layer on the metallic tungsten; and (e) exposing thetungsten layer to a nitriding agent to form at least a portion of atungsten nitride layer on the metallic tungsten.
 10. The method of claim9 wherein step (b) comprises depositing metallic tungsten by chemicalvapor deposition.
 11. The method of claim 9 wherein step (b) comprisesdepositing metallic tungsten by a pulsed nucleation layer process. 12.The method of claim 9 further comprising repeating step (b) and/or steps(c)-(e) to generate either a bilayer of W—WN or a multi-layeredstructure of multiple tungsten and tungsten nitride layers.
 13. Themethod of claim 9, wherein the ratio of W and N atoms are present in theW and WN layers in a ratio of approximately 2-to-1.
 14. The method ofclaim 9, further comprising forming a metallic tungsten plug on thetungsten nitride layer to form a tungsten interconnect, wherein thetungsten nitride layer serves as at least one of an adhesion layer, abarrier layer, and/or a nucleation layer for subsequent tungsten growth.15. The method of claim 9, wherein (c)-(e) form only a first portion ofthe tungsten nitride layer and further comprising: (f) exposing thesubstrate to a reducing agent to form a saturated layer of reducingagent on the metallic tungsten having the first portion of the tungstennitride layer formed thereon; (g) exposing the saturated layer ofreducing agent to a tungsten containing precursor to form a tungstenlayer on the metallic tungsten having the first portion of the tungstennitride layer formed thereon; and (h) exposing the tungsten layer to anitriding agent to form at least a second portion of the tungstennitride layer.
 16. The method of claim 15 wherein the reducing agent isa silicon hydride.
 17. The method of claim 10 in which tungsten nitridedeposition is carried out in a multi-station reaction chamber in which(i) CVD tungsten is deposited at one or more stations in the reactor;(ii) tungsten nitride is deposited at one or more deposition stations inthe chamber; (iii) the substrate is moved from one deposition station toanother such that the layered film of CVD-tungsten and tungsten nitrideis formed.
 18. The method of claim 12 in which tungsten nitridedeposition is carried out in a multi-station reaction chamber in which(i) PNL tungsten is deposited at one or more stations in the reactor;(ii) tungsten nitride is deposited at one or more deposition stations inthe chamber; (iii) the substrate is moved from one deposition station toanother such that the layered film of PNL-tungsten and tungsten nitrideis formed.